I went to a 295/70/17 tire, discount tire says diameter is 33.26, circumference is 104.49,and revs is 625.38. My calcs. say 606.37 revs.
I need somebody to check my math. I am recalibrating my speedo. with my XCAL2. Thanks

 

My guess is that the diameter and circumference is at a no-load state with maximum rated air pressure.
The circumference would be less under load which translates into greater revolutions per mile. Purely conjecture on my part, though.

 

606 is correct...you can check it at the following link:

http://www.miata.net/garage/tirecalc.html